/Engine/Plugins/Protocols/MQTT/Source/MQTTCore/Public/MQTTClientMessage.h
Get's the payload as a Json object, first converting to string. Returns false if unsuccessful.
bool GetPayloadAsJson ( [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [FJsonObject](API\Runtime\Json\Dom\FJsonObject) > & OutJson ) const